1. Field of the Invention
This invention relates to a process for preparing an electrode foil for use in aluminum electrolytic capacitors. This invention also relates to an electrode foil for use in aluminum electrolytic capacitors, which is prepared by this process.
2. Description of the Prior Art
Conventional aluminum electrolytic capacitors which are commonly used have been fabricated as follows. In the first step, on the coarse surface of aluminum foil which has been subjected to an etching treatment, a dielectric film is formed to yield an electrode foil. The second step consists of impregnating the electrode foil with an electrolytic solution for driving operations. Finally, the electrode foil so impregnated is wound together with a separator, resulting in an aluminum electrolytic capacitor.
In recent years, with increasing reliability and working life of electronic equipment, there has been a growing demand for aluminum electrolytic capacitors with high reliability, which are one of the important electronic parts.
Particularly, the capacitors used in charge-and-discharge circuits for welding machines, magnetizers, and the like, in which rapid charging and discharging are frequently repeated, as well as the capacitors used in inverter circuits to which an electric current with a high ripple percentage is fed, have to be driven under severe operating conditions, and therefore these capacitors are required to have extremely high reliability.
FIG. 2 shows a schematic structure of a conventional electrode foil for use in aluminum electrolytic capacitors which are used in the abovementioned charge-and-discharge circuits and high-ripple circuits. As shown in this figure, the conventional electrode foil comprises an aluminum base foil 1, an anodic oxide film 2 disposed on the base foil 1, and a porous oxide film 3 disposed on the anodic oxide film 2. This electrode foil is prepared as follows.
First, in an aqueous solution of oxalic acid, sulfuric acid, chromic acid, or the like, a constant direct current is allowed to flow between the aluminum base foil 1 and the counter electrode. By this anodic treatment, the porous oxide film 3 is formed on the surface of the aluminum base foil 1. Then, the base foil so treated is subjected to further anodic treatment in an aqueous solution of boric acid, adipic acid, or the like to form the anodic oxide film 2 between the aluminum base foil 1 and the porous oxide film 3, resulting in an electrode foil.
The anodic oxide film 2 formed by such a process has an amorphous structure in which there is no lattice mismatch as developed at the interface of crystals in the case of crystalline films. In general, the insulating characteristics of the amorphous film is less susceptible to thermal degradation because heat generation in the inside of the film can be reduced. Thus, the anodic oxide film 2 has a high withstand voltage, thereby attaining excellent resistance of electrolytic capacitors to charge-and-discharge and high ripple current.
However, in practical use, the electrolytic capacitors are driven at high temperatures for a long period of time. As a result, the anodic oxide film reacts with penetrated water which is contained in an electrolytic solution, thereby causing hydration on the surface of the film. Thus, insulating characteristics of the film is degrated by this hydration, so that the increase in leakage current eventually causes capacitor failure.